Effects of adjuvants on deposition efficiency of fenhexamid sprays applied to Chardonnay grapevine foliage

Adequate spray deposition on susceptible grapevine tissue is an essential requirement for effective chemical control of economically important diseases, such as grey mould, powdery mildew and downy mildew. The objective of this study was to evaluate the potential of some agricultural adjuvants to improve foliar spray deposition. Deposition quantity and quality was assessed by means of a spray assessment protocol using fluorometry, photomicrography and digital image analyses. The visual droplet rating technique developed by G Furness, Australia, was also included in initial assessments. Both assessment protocols showed that spray deposition quantity increased with increasing spray volume applications of 27 l ha−1 to 581 l ha−1 with a motorised backpack mistblower, but decreased at 698 l ha−1, possibly due to run-off. Addition of selected spray adjuvants at 526 l ha−1 volume demonstrated improved deposition quantity and quality. Agral 90 (ethoxylated alkylphenol), BB5 (acidifier), Nu-film-P (terpene oil), and Solitaire (silicone/plant oil) significantly improved deposition on upper and lower leaf surfaces compared with the fenhexamid-only and water sprayed control. Break-thru S240 (organosilicone) and Villa 51 (alkylpolyethylene glycol ether) did not improve deposition quantity, although remarkably better deposition quality was obtained. An adjuvant concentration effect (within the registered concentration range) was evident at spray volumes of 502 l ha−1, especially those retained on the upper leaf surfaces. Agral 90 and Nu-film-P effected significant improvement of spray deposition at the higher concentrations (18 ml and 50 ml hl−1, respectively), but not at the lower concentrations (6 ml and 20 ml hl−1, respectively). Solitaire improved deposition at the lower concentration tested (50 ml hl−1), whereas reduced deposition at the higher concentration (100 ml per hl−1) was attributed to excessive spray run-off. No significant improvement of spray deposition was observed for both concentrations tested with Villa 51 (50 and 100 ml hl−1). Spray mixtures with adjuvants Agral 90 and Solitaire yielded similar deposition values at 500 l ha−1 compared with the fenhexamid-only control at 720 l ha−1, but reduced deposition at the higher spray volume, possibly due to spray run-off. This study clearly demonstrated the potential of adjuvants to improve deposition quantity and quality, but highlights the necessity to match adjuvant concentrations and application volumes on the spray target to achieve maximum spray deposition.